1. Technical Field
This invention relates generally to submersible well pumping systems. This invention relates particularly to a positive displacement pumping system enclosed in a housing and comprising a multi-chamber hydraulically driven diaphragm pump, which uses a coiled tubing to simultaneously supply power and convey fluids.
2. Description of the Background Art
Hydraulically driven diaphragm pumps are positive displacement pumps which are nearly immune to the effects of sand in the pumped fluid because the pressure generating elements are isolated from the pumped fluid by a flexible diaphragm. In well pump applications, this type of pump is driven by a self contained, closed hydraulic system, activated by an electric or hydraulic motor where the pump, closed hydraulic system, and the motor are enclosed in a common housing and submerged in a well. There are many examples of this type of well pump in the patent literature, but currently none are in use as well pumps because of high cost and/or poor reliability. In well pump applications, the key design feature in pump systems is the method used to redirect or reverse the flow of working fluid from the fluid source, referred to as the auxiliary pump, to the working fluid sub-chamber. The reversal of the flow causes the pumped fluid to move into and out of a pumping fluid sub-chamber through check valves, accomplishing the pumping action.
U.S. Pat. No. 2,435,179 discloses a hydraulically driven diaphragm pump which uses a hydraulically actuated valve to reverse the flow of working fluid. The valve is driven by differential pressure between the fluid inside (working fluid) and the fluid outside (pumped) the working diaphragm. Normally, no differential pressure exists between the two volumes. The pump creates the differential pressure required to reverse the pump by forcing the diaphragm against the walls of the pumping chamber which has the disadvantage of creating diaphragm stress, which can lead to premature diaphragm failure. A more significant problem occurs in low volume applications. The nature of the pump requires that the hydraulically actuated valve be driven by the same pressure source controlled by the valve, which causes the valve driving force to be released when the valve transverses an intermediate position between states. In low volume applications, the valve can stop in this intermediate position before it has completely reversed the pump. This can cause the pump to either dither (rapid but incomplete movement of the working fluid in one direction) or go into a mode where half the flow is directed into each chamber, which causes the pump to stop functioning.
U.S. Pat. No. 2,961,966 discloses another method to reverse the flow of working fluid by reversing the direction of rotation of the electric motor driving the auxiliary pump. This patent discloses a method to sense the differential pressure between the working fluid and the pumped fluid to activate the electrical braking and reversal of the electric motor driving the auxiliary pump. This method also leads to diaphragm stress because differential pressure is required across the diaphragm to actuate the sensor. In addition motor reversal requires very complex electronics. Although theoretically possible, in practice the complexity of this method leads to high expense and unreliable operation due to the difficulty of controlling and reversing the electric motor in a downhole environment. To power this type of submersible pump, an electrical supply cable is typically used to connect the power supply at the surface to the electrical motor at the bottom of the well. Conventional submersible pump cables are armored with rubber or metal covers and are typically strapped to the outside of the production tubing as the pump is installed in the well. These cables, although armored, routinely suffer mechanical damage which results in cable failure. To better protect power cables and reduce costs, electrical cables have been placed inside coiled tubing and used to power and suspend submersible pumps in wells. A key design feature is a means of attaching the electrical cable to the inside of the coiled tubing to transfer the weight of the electrical cable to the coiled tubing to prevent the electrical cable from breaking under it's own weight.
U.S. Pat. No. 4,346,256 and U.S. Pat. No. 4,665,281 disclose two methods of suspending electrical cables inside of coiled tubing. In the field, these methods suffered from cable failures due to differential expansion of the various materials of construction. U.S. Pat. No. 5,146,982 discloses a method of overcoming this problem using a controlled spiral cable lay which allows for differential expansion. All of these cables are designed to work with high flow rate centrifugal pumps, consequently, the electrical cables and the hangers fill almost the entire cross section of the inside of the coiled tubing, which requires the output of the pump to be directed into the space between the coiled tubing and the well casing as opposed to between the coiled tubing and the electrical cable.
A significant problem which results from using positive displacement well pumps, such as sucker rod pumps, is sand and other solids which can cause premature pump failure due to excessive wear. Another significant problem is the expense and reliability of mechanical actuation systems used to power these pumping systems from the surface. Electrically driven submersible centrifugal pumps such as those used in most water wells, can be easily installed on coiled tubing and offer reliable service and economical operation but cannot be used in relatively low volume-high pressure applications because of clogging of small openings and unacceptably low efficiencies.
A pumping system, like the one disclosed herein, which combines the high reliability and ease of installation on coiled tubing of a submersible centrifugal pump with the high efficiency in low flow-high pressure applications of a positive displacement pump constitutes a significant advancement in the state of the relevant art. The invention disclosed in this application allows coiled tubing to be used to convey well fluid from the pump to the surface and allow the electrical power cable to be housed inside the same coiled tubing. The combination of functions of the invention is not currently possible, because achievable centrifugal submersible pump flow rates at the required pressures are too high to be compatible with commonly used coiled tubing diameters. In addition, mechanical actuation systems used in the sucker rod pumps disclosed in the relevant art are incompatible with coiled tubing.